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  1. #1

    STX 1100 Rebuild, advise/info wanted

    Hi all,
    1st post, Jet ski newbie but have plenty of bike engine experience.

    Had this STX for a month or so, usually flog it hard in the river and it runs sweet.
    Last weekend I took it out to Moreton Island for a cruise. (180km/ 110 Miles)
    As the ski isnt really good in open water, speed never got over 45 km's, under 5000 rpm and 1/4 throttle all day.
    On the last 30 km's of the journey (flat water section) I notice the ski was sluggish to get onto the plane, using excessive fuel and wouldn't rev out. It was obviously running for at least 1/2 hour with this damaged piston at 1/3 throttle, 4500 rpm

    When I go home I did a compression test, 120,120, 20 , yes 20, not 120
    The rear piston is damaged from the ring land upwards in line with the exhaust port.
    To my knowledge, the engine is standard with the exception of being 0.5mm over, CV carbs, premix, running 50:1 with Valvoline outboard oil.
    There was plenty of sand n the water jackets. It was debatable is the rear water jacket on the exhaust manifold was completely blocked or not. It was the rear piston that failed. Ive added some pics, would love to hear some opinions from people with experience.

    Question time:
    Its getting proX pistons 1.5mm over. I want reliability and fuel economy. Never say no to extra power, but its not a priority.
    What squish clearance would you recommend? And what head CC's/ compression ratio? As its a gas guzzler I'd like to be able to run 91 octane, happy to go to 95 if compression is increased and there are real gains to be had.
    If someone has a service manual, I'd like to know manufacturers specs for squish, deck heigh, piston to bore clearance and ring gap.
    Ideally I'd like to buy a service manual, but cant find one online.

    As it appears to be a lean mid rpm, low throttle failure, the finger points at the CV carbies. Should I spend the $$ getting them reconditioned of bin them and install the earlier non CV models? A mechanic I know has a set of pre CV 900 carbies that could work with a jetting change... thoughts?

    thanks in advance
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  2. #2
    Looks like you have overheated it.

    Pm sent

  3. #3
    steve45's Avatar
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    Keep in mind that the old-style carbs used an accelerator pump, which is difficult or impossible to get these days. I like the CVs myself.

    I"d recommend a coolant water strainer to keep trash out of the engine. Also, for long journeys, an electric bilge pump is very desirable.

  4. #4
    Thanks Steve, I think I'll stick with the CV's, will get a bilge pump too

  5. #5
    Does anyone have any advise/recommendations/experience with squish clearance?

  6. #6

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    Hit up ebay you can find a maintenance manual for under 10 bucks.

  7. #7
    Myself's Avatar
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    Notice how the yellowish color from fuel is missing on that one head and most of the piston top especially on the exhaust side is cleaned off. Water did that. I believe you blew an exhaust gasket possibly from overheating and warping the manifold. At the same time that would also allow a bit of extra air in really heating up that particular spot in the exhaust passage.

  8. #8
    Quote Originally Posted by Myself View Post
    Notice how the yellowish color from fuel is missing on that one head and most of the piston top especially on the exhaust side is cleaned off. Water did that. I believe you blew an exhaust gasket possibly from overheating and warping the manifold. At the same time that would also allow a bit of extra air in really heating up that particular spot in the exhaust passage.
    thanks for your input. The exhaust manifold was caked with sand so definitely plausible

  9. #9
    I just noticed your profile pic of it sitting on the beach.



  10. #10
    funny you say that, that pic was taken a few hrs before it died

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